Sulphate of divalent iron in the form of FeSO.sub.4.7H.sub.2 O, i.e. iron(II)sulphate heptahydrate, is a fatal product obtained, inter alia, in pickling baths used to treat iron and steel and based on sulphuric acid, and in the manufacture of titanium dioxide from titanium ores containing iron, primarily ilmenite, iron sulphate being removed from a sulphate solution by crystallizing precipitation by cooling. At present, it is necessary to dump large quantities of iron(II)sulphate heptahydrate, which, because this compound readily dissolves in water, creates a serious problem, and hence special safety precautions must be taken to prevent contamination of the surrounding water and groundwater. Further the physical properties of iron(II)sulphate heptahydrate are so poor that it is difficult to handle, since when handled it tends to agglomerate into a sticky mass which cannot readily be stored in normal storage plants, for example silo-type storage units. Neither can it be transported, dispensed or metered by means of conventional apparatus intended for use with solid, powderous products.
Even though iron(II)sulphate heptahydrate may have a high potential as a water-cleansing agent, it has been found that there is only a limited demand for this product by the technicians running the purification plants. This most probably due to the aforementioned problems encountered when handling the product. One way of making the product more attractive for use as a water-purifying agent is to heat it, to form a monohydrate, which has far superior physical properties compared to heptahydrate. Since large quantities of energy are required in the manufacture of such a product, this is far more expensive and thus does not create a positive solution to the problem of eliminating fatal iron(II)sulphate heptahydrate. Iron(II)sulphate heptahydrate products could also be used as a water-purifying agent in a solubilized state, thereby avoiding the aforementioned handling- and dosaging problems. It is not possible, however, for an iron(II)sulphate heptahydrate aqueous solution to contain more iron than about 2.9%, calculated on the weight of the solution, which is far from satisfactory with regard to transportation, since this means that large quantities of water must be transported together with the water-purifying iron sulphate. This increases the cost of transportation to such an extent that only those water-purifying plants located in the vicinity of the source of the fatal iron(II)sulphate heptahydrate products may be served economically.